1. Field of the Invention
The present invention generally relates to a light emitting device including an element (hereinafter referred to as an “organic EL element”) in which a structure is provided over a substrate and has a thin film layer (hereinafter referred to as an “organic EL layer”) that is made of an organic compound from which electro-luminescence (hereinafter referred to as “EL”) can be obtained and that is interposed between an anode and a cathode. More particularly, the present invention relates to a light emitting device in which an organic polymer (hereinafter referred to simply as a “polymer” although polymers with a polymerization degree of about 2 to 20 are called oligomers and polymers with a higher polymerization degree than that of the oligomers generally are called polymers) is contained in an organic EL layer and energy generated when a triplet excited state returns to a normal state (hereinafter referred to as “triplet excitation energy”) can be converted into emission light. In this specification, the light emitting device indicates an image display device or a light emitting device in which the organic EL element is used as a light emitting element. In addition, examples of the light emitting device also include a module with a tape automated bonding (TAB) tape or a tape carrier package (TCP) attached to an organic EL element, a module with a printed circuit board provided at the end of a TAB tape or a TCP, and a module with an integrated circuit (IC) directly mounted on an organic EL element by a chip on glass (COG) method.
2. Description of the Related Art
An organic EL element is an element that emits light by application of an electric field. In view of its characteristics such as light weight, low-voltage dc drive, and rapid response, the organic EL element has been receiving attention as a next-generation flat panel display element. In addition, since the organic EL element is of a self-light-emitting type and has a wide viewing angle, the organic EL element has been considered useful for display screens of portable devices.
In the organic EL element, an electron injected from a cathode and a hole injected from an anode are recombined to form an exciton, and the exciton releases energy to emit light when returning to the normal state. Possible excited states include a singlet state (S*) and a triplet state (T*). Their statistical generation rate is considered as S*: T*=1:3 (Reference 1: Tetsuo Tsutsui, “3rd Lecture Text, Organic Molecular Electronics and Bioelectronics Division, The Japan Society of Applied Physics” p. 31 (1993)).
In general organic compounds, however, no light emission (phosphorescence) is observed in the triplet excited state (T*) at room temperature. This is also true for the organic EL element. Usually, only light emission (fluorescence) in the singlet excited state (S*) is observed. Hence, the theoretical limit of internal quantum efficiency (a ratio of generated photons to injected carriers) in the organic EL element has been considered to be 25% on the basis of S*: T*=1:3.
Not all the generated light is released to the outside of the organic EL element. Part of the light cannot be extracted due to the refractive indices inherent to components (an organic EL film, electrodes, and a substrate) of the organic EL element. The ratio of light extracted to the outside of the organic EL element to the generated light is called light extraction efficiency. The efficiency is said to be about 20%.
For the reasons described above, it has been said that the theoretical limit of the ratio of photons that eventually can be extracted to the outside of the organic EL element to the number of injected carriers (hereinafter referred to as “external quantum efficiency”) is 25%×20%=5% even when all the injected carriers form excitons. In other words, even when all the carriers are recombined, mathematically only 5% of them can be extracted as light.
Recently, however, organic EL elements that can convert triplet excitation energy into emission light have been presented one after another and their high light-emitting efficiencies are receiving attention (Reference 2: D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “Improved energy transfer in electrophosphorescent devices”, Applied Physics Letters, vol. 74, No. 3, 442-444 (1999); Reference 3: Tetsuo Tsutsui, Moon-Jae Yang, Masayuki Yahiro, Kenji Nakamura, Teruichi Watanabe, Taishi Tsuji, Yoshinori Fukuda, Takeo Wakimoto and Satoshi Miyaguchi, “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center”, Japanese journal of Applied Physics, Vol. 38, L1502-L1504 (1999)).
A metal complex containing platinum as a central metal (hereinafter referred to as a “platinum complex”) is used in Reference 2, and a metal complex containing iridium as a central metal (hereinafter referred to as an “iridium complex”) is used in Reference 3. It can be said that both the metal complexes are characterized in that a third transition series element is introduced as the central metal. The metal complexes include those having a theoretical limit value of the above-mentioned external quantum efficiency of well over 5%.
As described in References 2 and 3, the organic EL element that can convert triplet excitation energy into emission light can achieve higher external quantum efficiency than that in prior art. In addition, emission luminance also increases with the increase in the external quantum efficiency. Hence, conceivably, the organic EL element that can convert triplet excitation energy into emission light plays an important role in the future development as an approach for achieving high luminous light emission and high light-emitting efficiency.
Materials forming the organic EL layer include low molecular weight materials and polymer materials when being broadly divided into two types. Here, the low molecule denotes a substance composed of monomers.
Since the above-mentioned platinum complex and iridium complex are monomers, they are included in the low molecular weight materials and usually are deposited by vacuum deposition. More accurately, it can be said that currently, there exists no deposition method other than the vacuum deposition. The vacuum deposition is advantageous in that a conventional shadow mask technique can be used when the film to be formed is to be patterned. In addition, there is also an advantage that the purity of materials can be maintained, since the vacuum deposition is a dry process in vacuum.
In the case of using the vacuum deposition, however, with respect to the material itself that is to be deposited, the amount of the material that is not deposited on a target but adheres to the inner side of a chamber is wasted. Hence, the vacuum deposition is not cost-efficient by no means. In addition, since the size of a substrate is limited in the conventional vacuum deposition technique, slight concern still remains with respect to the increase in area of the substrate.
Furthermore, for the low molecular weight organic EL layer formed by the vacuum deposition, a codeposition method is often used to dope the layer with a trace amount of pigment. However, there is also a disadvantage that it is technically difficult to codeposit a plurality of dopants with a host material at a time and besides at a constant rate.
On the other hand, there are some deposition methods in the case of using the polymer materials. Currently, the most convenient and general deposition method is spin coating. The spin coating has a disadvantage in that about 95% of a material is wasted, but has an advantage in that the increase in area can be achieved easily in the case of a monochromatic light emitting element produced using a single material.
With respect to doping, it is possible to carry out simultaneous doping with a plurality of dopants through proper preparation of solutions. In addition, it is also possible to carry out doping easily with pigments that cannot be used for doping by the vacuum deposition.
Furthermore, a precise patterning technique, which is difficult to be carried out by the spin coating, has been improved through the recent study and development of a technique of patterning a polymer material using an ink jet device (Reference 4: Japanese Patent Application Laid-open No. Hei 10-012377; and Reference 5 Japanese Patent Application Laid-open No. Hei 10-153967). When using this technique, the increase in area can be achieved more easily as compared to the case of the vacuum deposition of a low molecular weight material, and in addition, little material is wasted. Hence, in the future, there is a possibility that the polymer materials may become advantageous in terms of the increase in area and cost reduction.
Furthermore, the organic EL layer made of a polymer material (hereinafter referred to as a “polymer EL layer”) has advantages in, for example, being excellent in heat resistance and mechanical strength as compared to the organic EL layer formed using a low molecular weight material. The excellent heat resistance makes crystallization difficult to occur and thus allows devices to have high reliability. Moreover, in the future, if a film-shaped light emitting device with a flexible substrate is developed, high mechanical strength and reliability become important.
For the reasons described above, in the present situation, it is desirable to develop an organic EL element that includes a polymer EL layer and can convert triplet excitation energy into emission light. However, in organic EL elements including polymer EL layers, no emission light has been observed yet in the triplet excited state. Hence, under the present situation, the light-emitting efficiency of the organic EL elements with polymer EL layers is lower than the highest light-emitting efficiency of organic EL elements produced using low molecular weight materials.